1. Field of the Invention
The present invention relates to a polymer composition for forming in-situ composite molded articles having improved mechanical properties. More particularly, the present invention relates to a polymer composition for forming in-situ composite molded articles which comprises a polyphenylene sulfide matrix resin, a thermotropic liquid crystal polymer and a non-crystalline polymer.
2. Description of the Prior Art
Hitherto, general purpose plastics and engineering plastics have been reinforced with various kinds of inorganic additives in order to improve their mechanical strength and thermal properties. However, the use of such inorganic reinforcing agents causes some problems to the subsequent processing steps, for example, an excessive increase in the melt viscosity of the resulting polymer compositions, the abrasion of the processing machinery, and the like.
As an attempt to resolve the above problems, the so-called "in-situ composite forming technique" has been proposed, according to which technique, to an isotropic plastics such as general purpose plastics or engineering plastics as a matrix resin, an anisotropic, thermotropic liquid crystal polymer as a reinforcing agent is added. In this prior art technique, the thermotropic liquid crystal polymer reinforcing agent is present as a low viscous melt at the first stage, but it is crystallized upon cooling to form needle-like reinforced structures in the molding step. The technique of this type using the in-situ crystallization is advantageous since both the processability and the mechanical properties can be simultaneously improved.
A number of in-situ composite forming techniques are described in the prior patents and literatures. See "Polymer Engineering and Science," Vol. 27, p. 410; Vol. 29, pp. 9 and 600; and "Polymer Composite," Vol. 8, p. 158. In this prior art technique, non-crystalline engineering plastics are reinforced with thermotropic liquid crystal polymers. However, the resultant in-situ composite molded articles are still unsatisfactory in terms of their mechanical properties, heat-resistance, dimensional stability, etc..
In order to remove these defects, it has been suggested to use, as the matrix resin crystalline engineering plastics, for example, polybutylene terephthalate (PBT), nylon, and polyphenylene sulfide (PPS) in place of the non-crystalline plastics. For example, U.S. Pat. No. 4,202,951 discloses a moldable blended composition comprising polyphenylene sulfide in admixture with branched polyphenylenes. The blend exhibits superior mechanical properties after heat aging to either component alone. U.S. Pat. No. 4,710,546 discloses a polyphenylene sulfide reinforced with chemically activated fibres or fillers. This reinforced polyphenylene sulfide is useful as an embedding composition. European Patent No. 0 103 279 discloses a poly(arylene sulfide) having a lower melt crystallization temperature. The molded objects obtained from such arylene sulfide polymers exhibit improved physical properties.
However, in order to obtain successful results from the prior art techniques mentioned above, the two or more crystalline polymers to be blended must be properly selected taking into account the physical properties thereof. Generally, when a crystalline polymer is blended with other crystalline polymer, if the two polymers fail to form co-crystallized structures, then the resulting blend tends to give rise to phase separation in the crystallization step thereof, resulting in poor mechanical properties. In practice, this tendency becomes serious when the crystallization temperature, crystallization rate and thermal expansion coefficient of the two polymers are different from each other or when the interface adhesiveness between the two polymers is poor. From this point of view, the thermotropic liquid crystal polymers used in the prior art are difficult to form a stable interface with the crystalline matrix polymer due to their rapid crystallization rate and the negative volume change exhibited during the heat treatment. Actually, with an increase of the content of thermotropic liquid crystal polymer in the polymer blend, the mechanical properties of the molded articles become lower accordingly. These problems encountered in the prior art using thermotropic liquid crystal polymers have been reported in "Plastics Molding Techniques," Vol. 4, p. 11.